Speaker
Mr
Javier Alberola-Perales
(Airbus DS - Crisa)
Description
The Mars 2020 mission includes a rover designed to investigate key questions about the habitability of Mars, and assess natural resources and hazards in preparation for future human expeditions. The mission is part of NASA's Mars Exploration Program, a long-term effort of robotic exploration of the Red Planet.
Mars Environmental Dynamics Analyzer (MEDA) is one of the Mars 2020 rover instruments being developed for the mission. MEDA will include a sensors suite to provide measurements of Mars near-surface atmosphere and ground temperatures, wind speed and direction, pressure and relative humidity. It includes also a sky pointing camera a set of photo-detectors for sky imaging and measurement of ultra-violet, visible and near-infrared irradiations at several bands allowing characterizing the atmospheric dust profile.
MEDA wind sensor data acquisition will require the use of mixed-signal electronics to implement the front-end interface for the wind sensor transducers. These sensors are located around the Remote Sensing Mast (RSM) of the Rover. If the electronics is near to the transducers, and remotely connected to the rover’s Instrument Control Unit (ICU) through a simple serial link, the harness is notably reduced, saving a significant amount of mass. However, if the mixed-signal electronics is near to the transducers, it will be exposed to the Martian extreme temperatures, between -128ºC to +50ºC. The problem is not only the temperature range per se, but the fact that for a given sol, the temperature excursion can be of more than 70 to 100 degrees, so, when accumulated during all the mission (1.5 Martian years equivalent to 3 Earth years) all materials suffers from extreme wear-out and fatigue. The application (the ASIC) must be also demonstrated to withstand three times the mission life, that is, 3015 thermal cycles.
This precludes the use of conventional space qualified semiconductors, which are typically down limited to -55ºC and not designed for withstanding those thermal cycles. To overcome this challenge, a mixed-signal ASIC with an operating temperature range of -128ºC to +110ºC has been developed. The ASIC need also to be packaged using specific materials and processes designed to counteract that fatigue. This was the case also of the previous ASIC developed for the REMS instrument on board Curiosity Rover. In this case, the REMS ASIC was tested to over 10.000 thermal cycles without showing any functional, electrical or mechanical degradation. The experience and heritage taken during the REMS ASIC development have been applied to the MEDA ASIC from the beginning to define the ASIC functionalities, the technologies and the verification program.
The wind speed and direction are detected by the Wind Sensors using sigma-delta control loops. A wind sensor comprises four dice, each one with a temperature detector and a heater. The sigma-delta loops force the four dice to reach the same temperature, by applying the necessary power to each heater. Depending on the wind speed and direction, the loops will have to apply more power to one heater or another. Thus, by knowing each heater’s applied power is possible to calculate the wind speed and direction in one axis. This ASIC implements twelve control loops to interface with three wind sensors, one per axis.
In addition to acquire the wind sensors information, the ASIC includes up to nine analog channels to interface other type of sensors, like thermocouples, thermopiles or resistance temperature detectors (RTDs). This enhances the ASIC front-end capabilities, expanding the applications range, and covering possible unexpected needs in MARS 2020 or in other missions.
A digital state machine controls the wind sensor loops and analog acquisitions. It also communicates with the ICU through an UART interface, receiving configuration data for the different acquisition modes, and transmitting the wind sensors and analog acquisitions digitized data. Also, if a SEU is detected, it is reported to the ICU through this serial channel.
MEDA WS FE ASIC main features:
• 12 sigma-delta control loops for three wind sensors. 14-bit resolution for 0.5Hz and 1Hz acquisitions, and 13-bit resolution for 2Hz acquisitions.
• 9 analog channels (switchable gain preamplifier + 15-bit ADC) with internal calibration, to acquire RTDs, thermocouples and/or thermopiles.
• Digital machine to configure and control the wind sensor and the analog channels, with SEU detection.
• 19200 baud UART with RS-422 interface.
• Over-temperature protection for the ASIC and the wind sensors.
• Internal housekeeping telemetries: Junction temperature and supply voltage.
The ASIC design have been developed by the Instituto de Microelectronica de Sevilla (IMSE) and Crisa, using AMS 0.35 process and ECSS-Q-ST-60 methodology. We used rad hard by design libraries, pre-developed by IMSE, and fully characterized in temperature to -110ºC. ASIC prototypes (full functionality) will start testing in February 2016. Once validated, we will go for a second design to foundry iteration, to fine tune and improve functionalities and to package the ASICs using the final high reliability package. The ASIC will go through a full screening and lot qualification process afterwards.
Summary
In this paper proposal, a mixed-signal ASIC is presented, to be used in the rover of the Mars 2020 NASA mission. The ASIC is part of one of the rover’s scientific instruments: the Mars Environmental Dynamics Analyzer (MEDA). The ASIC’s main purpose is to digitize the information of the Wind Sensors and send it to the Instrument Control Unit (ICU) through a simple serial link (UART), so reducing the harness and saving a significant amount of mass. This forces the ASIC to be next to the wind sensors, and so exposed to the extreme Martian temperatures, between -128ºC to 50ºC with daily cycling during many years. The operation within such extended temperature range without degrading the performances presents a big challenge to the ASIC design.
Primary author
Mr
Jose F. Moreno-Alvarez
(Airbus DS - Crisa)
Co-author
Mr
Javier Alberola-Perales
(Airbus DS - Crisa)
